>>That could happen if the bird flu virus got into an animal,
most probably a pig, which can harbor the kind of influenza virus that
affects people, mate and mutate. There is not yet any sign that has
happened <<

There are VERY clear signs
that dual infections have happened, particularly in the isolates from
Vietnam and Thailand. Isolates from both countries have
polymorphisms not found in other H5N1 isolates. The polymorphisms
however, can be readily detected in mammalian isolates from pigs and
humans. The virus mates (via dual infection) and mutates (via
recombination) on a very regular basis, and the mating is quite
evident in the isolates from fatal human cases in Vietnam and
Thailand.